"No cell phones at Prom" legal?

Its not a matter of being 'legal' or not, its just the rules. Asking if its legal is a little far-fetched.
It may not be the schools rule, it could be the dj's etc as mobile phones can interfere with electrical appliances.
If you put your mobile too close to the television the tv starts making noises and messes up a little, well imagine what it would be like with 100+ mobiles surrounding it. It is exactly the same with a sound system. The rule has been put into place so that you have a better prom without any mistakes or mishaps that could ruin it for you.
They're not saying you cannot take pictures or anything, you can still take a camera.

I am sure you can handle not posting pictures of yourself on Facebook for a couple hours.